The invention relates to an irradiation device for irradiating a substrate by a UV emitter, comprising the process steps of:                (a) operating the UV emitter at a nominal operating radiation power that is a function of a nominal operating temperature;        (b) continuously feeding the substrate, at a feed rate, into an irradiation field that is defined by the UV emitter; and        (c) irradiating the substrate in the irradiation field.        
Operating processes of this type are often used for operation of irradiation devices in flow-production, for example for disinfection, water reprocessing, or hardening of lacquers, adhesives or plastics.
Known irradiation devices are provided with one or more UV emitters as the radiation source. UV emitters in this spirit include, for example, mercury vapor low pressure lamps, medium pressure lamps, or high pressure lamps. The UV emitter or emitters are arranged appropriately in the irradiation devices, such that they define an irradiation field, in which the substrate is irradiated at a pre-determined minimal irradiation strength. The substrate is fed into the irradiation field by a conveying device, wherein it passes through the irradiation field at constant speed, if possible.
For a given irradiation power of the UV lamp, the residence time of the substrate inside the irradiation field defines the irradiation energy hitting the substrate. A regulation of the conveying rate of the substrate can be used to adapt the irradiation energy hitting the substrate to the ongoing irradiation process.
It is desired, as a matter of principle, to provide for the operation of the irradiation device to be as continuous as possible, i.e. uninterrupted operation, in order to attain good energy efficiency. Upon any interruption of the production process, it must be ensured that a substrate that remains in the irradiation field is not being damaged by excessive irradiation.
To prevent the substrate from being damaged, the UV emitters can be switched off if the production process is interrupted. However, the emitters take a certain time to reach their nominal radiation power again after being switched on. In this context, the radiation power of the UV emitters depends mainly on their temperature. After a cold start, the UV emitter warms up steadily until it reaches its operating temperature. The radiation power stays constant only once the operating temperature is reached. The period of time until the operating temperature is reached is called heating time. Usually, the heating time is on the order of several minutes. Therefore, a re-start of the UV lamp is usually associated with a delay of the production process.
In order to ensure a short heating time after interruption, the prior art refrains from switching-off the UV emitters. Rather, the use of a shielding element interrupting the beam path between UV emitter and substrate is proposed, such that the operation of the UV emitter can be continued even in a standstill of the production process without this having a direct effect on the substrate.
An irradiation device of this type is known from Japanese patent application JP 06-056 132 A. The irradiation device comprises a disinfection lamp that defines an irradiation field, as well as a conveying device that conveys the substrate through the irradiation field. In order to prevent excessive irradiation of a substrate that remains in the irradiation field during a standstill of the irradiation device, the invention of the Japanese publication proposes to arrange, between the UV disinfection lamp and the substrate, a closure door (shutter) that interrupts the beam path between the UV disinfection lamp and the substrate, in the case of a standstill of the production process.
However, the shutter is disadvantageous in that it absorbs some and reflects some of the radiation emitted by the UV emitter, such that it can contribute, in turn, to strong local heating of the surroundings of the UV emitter and thus to heating of the UV emitter. The UV emitter being heated excessively can not only impair its radiation power, but it also contributes to the ageing of the emitter, which is associated with a decrease in the emission in the UV range and a reduced service life of the emitter.
Moreover, continuing operation of the UV emitter during a prolonged standstill is associated with energy being consumed and, often, the substrate to be treated being damaged.
Moreover, the use of a shutter requires the availability of a certain amount of installation space, i.e., sufficient distance between the emitter and the substrate. However, the distance reduces the irradiation strength. As a matter of principle, the irradiation strength attained is largest if the distance between emitter and substrate is as small as possible.
Finally, a shutter is a moving component that needs to be triggered and has a certain susceptibility to failure.